The rapid growth of non-injecting heroin use (NIU) has serious consequences for controlling injecting drug use (IDU), the spread of Human Immunodeficiency Virus (HIV) and other blood born pathogens, since IDU is a major source of illness and death among drug users, especially among US racial and ethnic groups. Mexican Americans are experiencing this increase among a drug using population that has consistently been characterized by high rates of drug injecting and AIDS risk behavior. In this study, we propose to examine the roles of social networks by using a "network facilitation" approach and cultural and contextual variables associated with transitioning from NIU of heroin to injecting and in the transmission of HIV, HBV, and HCV among a sample of NIU Mexican American heroin users in San Antonio. Social networks may facilitate the process that determines the extent to which an individual who is susceptible for transitioning to injecting or for seroconverting transitions to injecting and becomes infected. The study design will include a cohort of 300 Mexican American NIUs that will be interviewed and tested for HIV, HBV, and HCV at baseline and at three follow-ups at 6-month intervals. The design also contains an ethnographic component that includes observations and interviews with three cohorts of 20 subjects and follow-up interviews twice each year through the fourth year. The aims of the study are to determine: 1. the rate of transitioning to IDU and predictors of transitioning, including networks, risk behaviors, adherence to cultural values and norms, and personal susceptibility; 2. the extent of, predictors of, and changes in high-risk sexual behavior and networks and the extent to which female NIUs are at elevated sexual risk; 3. the extent of, predictors of, and changes in high-risk injecting behaviors and networks among those transitioning to injecting; 4. the extent of and predictors of prevalent (HIV, HBV and HCV) infection; 5. the incidence rate of HIV, HBV and HCV and predictors of seroconversions; and 6. the nature and influence of cultural and contextual variations on transitioning to injecting and on HIV, HBV, and HCV prevalence and incidence. This research will contribute to existing knowledge by providing new data on the role of networks and social processes by which NIUs transition to IDU and high-risk behaviors associated with HIV, HBV, and HCV. The research will provide information that can be used to develop network-based interventions that can be conducted not only among couples and friendship groups, but also at the community-wide level through the diffusion of norms and practices among NIUs to prevent the transition of injecting and other harmful health consequences of non-injecting heroin use.